Clegg has a chance to prove the Lib Dems won't be wiped out in 2015 and Cameron a chance to show how the Tories could win a majority.

For the first time since the coalition was formed, the governing parties will do battle in a Lib Dem-Tory marginal. Photograph: Getty Images.

The resignation of Chris Huhne as an MP heralds perhaps the most politically interesting by-election of this parliament. His Eastleigh constituency, just outside Southampton, is that most intriguing thing: a coalition marginal. His majority in 2010 was 3,864 over the Conservatives, with Labour a distant third. While the Guardian's Jonathan Freedland yesterday, set out the "problem for all parties" this by-election poses, it also presents some great opportunities.

Ipsos MORI’s latest poll in January had support for the Liberal Democrats at the lowest level recorded since 1990, on just eight per cent, with UKIP on nine per cent, the highest rating we have ever recorded. At the other end, Labour had a 13-point lead over the Conservatives. Of course, this is the national picture and locally things can be very different, but it does illustrate the low ebb from which the Liberal Democrats will begin the by-election campaign.

This will be the first time since the formation of the coalition that the two parties will seriously do battle against each other for a Westminster seat. It may well prove a dry run for the general election as we get to see just how the two governing parties will set out their stalls.

The Lib Dems have held Eastleigh since 1994 when they took the seat from the Conservatives in, appropriately enough, a by-election. At the four general elections since then, the Liberal Democrat majority over the Conservatives has never been more than 4,000. It was high on the Conservatives’ list of target seats in 2010 and will be again in 2015.

The opportunity then presents itself for the Tories to show that they can win seats from the Lib Dems at the next general election. If David Cameron and his party are to win a majority, it is seats like Eastleigh that will need to turn blue. The spoils of victory would bring a much-needed morale boost to the party and help to settle recent rumours of challenges to Cameron's leadership.

The opportunity for the Lib Dems is also great. Winning a closely fought by-election against the Conservatives would be a huge boon to a party apparently struggling so badly in the polls. It would provide a shot in the arm for members, activists and their leaders. Nick Clegg and his team would have proved that the Liberal Democrats have not been wiped out by forming a coalition with the Conservatives and that they can hold seats at the general election.

For Ed Miliband and Labour the temptation must be to sit back, save valuable resources and watch the Tories and the Lib Dems tie themselves in knots campaigning against each other.

Since the 1994 by-election, when Labour came second, they have been less and less competitive in Eastleigh. In 2010, they won just 9.6 per cent of the vote (compared to the Liberal Democrats’ 46.5 per cent). However, think what a good performance, however unlikely, could do for Labour. Even coming second, as they did in 1994, would surely have Conservative and Liberal Democrat strategists staring at the 2015 drawing board. Miliband, a leader who has not quite convinced his own party's voters yet (only 53 per cent of Labour supporters are satisfied with his leadership compared to 75 per cent of Conservatives for Cameron), would show Labour has gained ground in areas where it was previously uncompetitive.

In that 1994 by-election a little known Nigel Farage won just 169 more votes than Screaming Lord Sutch. Farage, now leader of UKIP, is being described as the "wildcard" in the 2013 by-election and will undoubtedly have a bigger impact on the results this time around.

But the UKIP leader has yet to decide whether he will run. The perils are obvious, if he runs and loses heavily it would slam the brakes on the momentum he and his party have been building over the last few years. A successful campaign – which does not necessarily just mean victory – would, by contrast, put further wind in the UKIP sails ahead of the general election. Farage may decide the personal risk is too great but still fully back another UKIP candidate. The opportunity for UKIP is to demonstrate that they remain a growing force in British politics.

The prospect of a strong UKIP campaign adds another dimension to the race, and allows political watchers to see the effect it can have on the other parties. We know that 43 per cent of UKIP voters voted for the Conservatives in 2010. So, will UKIP be successful in persuading Conservatives to actually vote for them and in large enough numbers to impact on the result? It is also the first electoral test for the Tories since Cameron’s promise of an in/out EU referendum. How will that impact on UKIP’s support, and what they campaign on? A damp squib of an election for UKIP could well vindicate Cameron’s decision to make the pledge.

In 2010, UKIP won just 3.6 per cent of the vote, but by-elections are unusual and can produce surprise results. Would George Galloway have won in Bradford West last year had it been a general election? Probably not, and it does not necessarily point to further success for the Respect Party in 2015. All of which throws up the prospect of another "unusual" result in Eastleigh which tells us nothing about the next general election.

This by-election is different though. It may not tell us who will win the general election but it will help to shape the narrative for all the parties, either inflating or deflating their political balloons, however temporarily. It will also give us a glimpse into some of the more intriguing aspects of 2015: how will the coalition parties fight each other? What impact will UKIP have on the outcome? Fortune though, favours the brave. There is a great deal to be won in Eastleigh for those who want it.

N.B. It is worth noting that this by-election came about because Chris Huhne asked his wife to take his speeding points; in 2006, 12 per cent of UK drivers said they would ask a friend of relative to take speeding points for them if they were facing a ban.